Edward Gibbon hates Constantine. He sees Constantine's reforms as "the mortal wound which had been so rashly or so weakly inflicted...". Interestingly, though, he devotes a huge swath of his History to Constantine, more than any other period in Roman history. This is no doubt because he saw the main causes of the fall of the Roman Empire as barbarism and Christianity. He wrote, burned, and then rewrote this section; he writes,"it is difficult to arrange with order and perspicuity the various transactions of the age of Constantine: and so much was I displeased with the first Essay, that I committed to the flames above fifty sheets." (Memoirs, p. 159). Thus it seems he experienced some frustration when writing the section.
Serious issue is taken with the so-called conversion of Constantine; he regards the two main sources for the event, Eusebius and Lactantius, as next to worthless. The traditional tale runs as follows: prior to the battle of the Mulvian Bridge, Constantine saw a vision in the heavens, which was seen again in a dream of his. In this dream, the emperor was ordered to place the sign on his banner, and fight under the auspices of the Christian God. Constantine proceeded to win the battle.
Gibbon writes, "I shall endeavour to form a just estimate of the standard, the dream, and the celestial sign; by separating the historical, the natural, and the marvellous parts of this extraordinary story, which, in the composition of a specious argument, have been artfully confounded in one splendid and brittle mass." (Decline and Fall XX, 317) He begins first with Lactantius. Of his account Gibbon notes that it was published "at Nicomedia about three years after the Roman victory" which afforded "ample latitude for the inventions of declaimers, the credulity of party, and the tacit approbation of the emperor himself." (Decline and Fall XX, 321). Thus Gibbon seems to think that the account of Lactantius is mere fabrication and that Constantine perhaps had some hand in the creation of the story.
Turning his guns to Eusebius, Gibbon then proceeds to demolish the Church historian's account. He notes that Eusebius only makes mention of Constantine's conversion in his De vita Constantini, and not in his Ecclesiastical History, which was published earlier than the Life of Constantine; in his own words, "the silence of the same Eusebius, in his Ecclesiastical History, is deeply felt by those advocates for the miracle who are not absolutely callous. " (Decline and Fall XX, 323) He then argues that "the advocates for the vision of Constantine are unable to produce a single testimony from the Fathers of the fourth and fifth centuries." Indeed, Gibbon notes that there is no independent testimony from any witnesses of the event in Eusebius, or anywhere else, save Lactantius, and that the sources for both of these accounts was Constantine himself. He takes this to mean that the whole event is a sham, which was fabricated by Constantine.
Why, then, according to Gibbon, would Constantine create such a story? Because he “used the altars of the church as a convenient footstool to the throne of the empire,” (Decline and Fall XX, 314), that is to say, Constantine used the Church to further his position and this story of his conversion served as a propaganda tool. Christians, though a minority, were slowly becoming a powerful force in the Empire, and Constantine, according to Gibbon, recognized this and acted out of a sheer desire for practical advantage; “In the beginning of the fourth century the Christians still bore a very inadequate proportion of the inhabitants of the empire; but among a degenerate people, who viewed the change of matters with the indifference of slaves, the spirit and union of a religious party might assist the popular leader to whose service, from a principle of conscience, they had devoted their lives and fortunes.” (Decline and Fall, XX, 316). Gibbon states later that, “the same motives of temporal advantage which might influence the public conduct and professions of Constantine would insensibly dispose his mind to embrace a religion so propitious to his fame and fortunes.” (Decline and Fall XX, 325)
Thus, Gibbon regards the story of Constantine’s conversion as part of his shrewd operation to gain practical advantage from a conversion to Christianity, and does not hold any real historical weight.